Ge Nanocrystals Grown on Si(111) by Molecular Beam Epitaxy with and without CaF 2 Buffer Layers
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ABSTRACT The Stranski-Krastanov growth mode of Ge thin films on Si and the clustering behavior of Ge on calcium fluoride have been exploited to grow self-assembled nanocrystals by molecular beam epitaxy. The growth of the samples was monitored in situ with RHEED, and they were analyzed ex situ with AFM and RBS. For each system (Ge/Si and Ge/CaF 2/Si), the dependence of Ge islanding on substrate temperature and on substrate misorientation was studied. When grown on Si(l 11) at temperatures between 500'C and 700°C, Ge clusters nucleated at step edges on vicinal wafers and nucleated homogeneously on on-axis wafers. Above 600'C, no transition to a spotty REJEED pattern, which would be expected for island growth, was observed for the vicinal samples. Ge grown at 500°C on on-axis Si(l 11) formed islands with a relatively narrow size distribution, typically 160nm in diameter.and 10nm to 20nm in height. When grown on a CaF 2 buffer layer, Ge islands nucleated homogeneously at a substrate temperature of 750'C, resulting in randomly distributed, oblate crystallites approximately 100nm to 200nm in diameter. At 650 0 C and 700 0C, although we still observed many randomly distributed, small crystallites, most islands nucleated at step bunches and had a length scale of over 500rnm. INTRODUCTION Molecular beam epitaxy (MBE) remains a promising tool for the growth of self-assembled semiconductor structures with quantum confinement in three-dimensions. The challenge remains to grow an array of such nanocrystals with a narrow size distribution (peaked around a few tens of nanometers) with good crystal quality and well understood surfaces. The growth of Ge on Si, primarily Si(100), has been widely studied over the last few years, and the growth mode is known to be Stranski-Krastanov (SK). That is, Ge deposits pseudomorphically for three to six monolayers until the strain energy in the film is large enough for dislocation formation to be favored, and further growth is in the form of three-dimensional islands.' However, in the initial stages of their formation these islands may be dislocation-free and coherently strained to the substrate. 2 Coherent islanding may be a general feature of SK growth and has also been studied 3 in the InGaAs/GaAs system and has been exploited to grow uniform quantum dots. 4 The uniformity may be related to the technique of repeated growth interruption. 5 A study of Ge islands with UHV-TEM indicates that, after the development of the first strain-relieving dislocation, the islands grow primarily vertically, except for "spurts" of lateral growth which occur with the introduction of each successive dislocation. 6 The aim of the present study is to control the transition to island growth and the subsequent growth of islands to 139 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
achieve uniformly sized quantum dots. The growth of Ge on CaF2 has been studied primarily with a view toward growing twodimensional Ge layers, rather than islands. When grown on vicinal substrates, CaF2 forms regu
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